Contract sample language/Selling precious metals for Bitcoin

Date

The offer should be dated, since PGP signing doesn't establish the date.

December 14, 2012.

Offer for a fixed amount

An offer for a fixed amount is simple to understand and doesn't require the buyer to worry about volatility.

I, insert name here, hereby offer to sell ten (10) one-ounce Austrian Philharmonic gold coins as described below,
for 132 Bitcoins each per coin, which includes fully insured Registered Mail shipping to any US address.
The coins can be bought in increments of a single coin - in other words, you do not have to buy all ten.

Offer for an amount relative to MtGox

I, insert name here, hereby offer to sell ten (10) one-ounce Austrian Philharmonic gold coins as described below,
for the Bitcoin equivalent of $1770 per coin, which includes fully insured Registered Mail shipping to any US address.
The coins can be bought in increments of a single coin - in other words, you do not have to buy all ten.

Any or all of the following conditions can be made part of the offer. It will make the offer harder to accept, but also lessens the possibility of an opportunistic offer based on an unexpected surprise from MtGox.

The following restrictions apply to this offer.  If an attempt to accept this offer is made while invalid for a reason
listed here, I reserve the right to elect to either issue a refund or complete the transaction regardless, without
prejudice to my right to refuse to do so under the same conditions in the future.
* This offer is only valid during periods of low volatility, defined as follows: the 24-hour MtGox high is less than
   105% of the 24-hour MtGox low.  
* This offer is only valid so long as the MtGox data feed for the last 24 hours is available and believed to be
   substantially accurate.
* This offer is only valid so long as the USD/BTC exchange rate is at least $13.50.

Describe what's for sale

The 1 ounce Austrian Philharmonic gold coins for sale are brand new in an unopened mint tube of 10.
If a single buyer purchases all ten coins, they will be shipped in the original unopened mint tube. 
More about the coin: http://www.monex.com/prods/gold_vienna.html
I certify that these coins were acquired through a reputable precious metals dealer.

Describe how to accept the offer

If requesting bitcoins to be sent directly to you:

The Bitcoin address for this transaction is: insert bitcoin address here
See activity: http://blockchain.info/address/insert bitcoin address here

If this is an address under the control of an escrow agent:

This Bitcoin address is under the combined control of me and an escrow agent as described below.
A PGP-signed message from the escrow agent follows my message.

For a fixed price offer:

As long as this address has fewer than 1320 Bitcoins sent to it, my offer is valid and unclaimed.

If not for a fixed price:

I anticipate that if this offer is accepted, that the above Bitcoin address will have had at least 1200 bitcoins
sent to it.

To accept this offer, send bitcoins in an even multiple of 132 bitcoins, but first:

Add a cancellation clause if necessary

It is totally reasonable to be willing to cancel your unaccepted offer when necessary. One way to cancel your offer is just to send enough of your own bitcoins to the address, essentially satisfying it yourself. But if you can't do that:

I reserve the right to cancel this offer so long as it has not been accepted.  I will signal that I have
cancelled my offer by sending 0.01 BTC from address XXXXXXX to the above address.  Any payment
received after cancellation will be refunded.

Add instructions

1. Please check the address to ensure that the offer has not already been claimed in its entirety. 
2. You must be able to prove you are the owner of the Bitcoin address(es) that sent the payment.
Either you need to be able to sign messages with it, or move an arbitrary number of coins sent to it
upon request.
3. You must be able to accept Registered Mail at an address in the US.
4. The first person(s) who pay the above address (as determined by the BlockChain.info first-seen 
timestamp) are the person(s) to whom I am selling.  If I receive payments to purchase in excess of the ten
offered coins, I reserve the right to either refund the excess payments, or to sell further coins at the same
price, at my option.  The ten coins will be allocated in the order the payments for them arrive.
5. This offer is for gold coins at a fixed price.  Any payment that is not a multiple of 132 BTC in a single transaction
will not be deemed an acceptance of this offer.
6. This offer unconditionally expires at 18:00 UTC on December 16, 2012 to the extent it is not accepted
before then.
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Helpful info

Stay safe! Always verify the PGP signature of any signed offer asking you to send bitcoins. Making a copy of the signed offer is a good idea too. My PGP key is available from my bitcoin-otc profile. http://bitcoin-otc.com/viewratingdetail.php?nick=casascius&sign=ANY&type=RECV